The present invention relates to a novel cationic electrodeposition paint composition and a process for preparing the same, and more particularly to a cationic electrodeposition coating composition in which the electrodeposited coating film of the electrodeposition paint composition has an epoxy-acrylic double-layered structure, and in which an organic solvent content can be minimized, and a process for preparing the same. Further, the present invention relates to a pigment grinding vehicle suitable for electrodeposition coating of the cationic electrodeposition paint composition.
The cationic electrodeposition paint is widely used for primer coating of cars, household electric appliances, and industrial machines. Recent research at home and abroad on a cationic electrodeposition paint have concentrated on paints related to energy saving, safety and environmental considerations. Also, research or binders of electrodeposition coating have concentrated on developing high functional and high efficient electrodeposition paint having various purposes and characteristics. Among the research, studies on reducing an organic solvent emissions from paint operations are being developed. The motive for these studies is the regulation of emission of organic solvents for the protection of the environment. As examples of regulations related to the regulation of emission of organic solvents, a regulation for volatile organic compounds (VOC) of the Environment Office in U.S. and an atmospheric purification law in Germany can be illustrated. The former establishes a guideline for the amount of organic solvents in a paint, while the latter restricts the amount of organic solvents discharged from the paint used for coating a car. Since 20% of the amount of carbon dioxide discharged in the whole world are due to the coating industry (for example, organic solvent and combustion by means of a drying oven), the coating industry is a major cause of environmental pollution. Hence, the coating industry has become the subject of supervision. Presently, as an electrodeposition coating for primer coating, epoxy cationic electrodeposition paints are mainly used. However, these epoxy cationic electrodeposition paints are rich in organic solvents and these solvents are evaporated from the electrodeposition tank, thereby emitting an offensive odor in a workpiece. Also, these solvents are emitted during baking, so environmental pollution problems occur. Though these epoxy cationic electrodeposition paints have excellent properties such as chemical resistance, corrosion-resistance, adhesion etc., they are lacking in properties such as weather-resistance, yellowing resistance, etc.
Accordingly, considering the problems described above, it is an object of the present invention to provide an improved cationic electrodeposition composition in which an organic solvent content can be minimized and the electrodeposited coating layer has an epoxy-acrylic double-layered structure for improving properties such as weather-resistance, yellowing resistance, etc.
It is a further object of the present invention to provide a process for preparing a cationic electrodeposition composition particularly suited for manufacturing cationic electrodeposition coating.
It is yet another object of the present invention to provide a pigment grinding vehicle suitable for electrodeposition coating of the cationic electrodeposition composition.
In accordance with the present invention, a cationic electrodeposition coating composition having a resinoid dispersed into an aqueous medium is provided. The cationic electrodeposition coating composition of the present invention essentially comprises a cationic electrodeposition resin prepared by a reaction of (a) 40-60 percent by weight of a cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction, (b) 5-10 percent by weight of an acrylic cationic electrodeposition resin having an amino group, (c) 1-3 percent by weight of a fatty acid ester resin synthesized by an estrification reaction of styrene-allylalcohol copolymer and a fatty acid, and (d) 30-50 percent by weight of a blocked polyisocyanate crosslinking agent. A cationic electrodeposition coating composition is obtained by dispersing the cationic electrodeposition resin into an aqueous medium. At this time, an organic solvent is contained less than 2.0 percent by weight based on the dispersed solution of the cationic electrodeposition resin.
Hereinafter, the present invention will be explained in more detail.
The cationic electrodeposition resin according to the present invention is a major film forming resin in an electrodeposition coating composition. Some of these known film forming resins are illustrated in U.S. Pat. Nos. 3,663,839; 3,984,299; 3,947,338; and 3,947,339. These film forming resins represent a resin having an amino group formed by an addition reaction of polyepoxide with a primary, secondary or tertiary amine. As for the cationic electrodeposition synthetic resin formed by the addition reaction of polyepoxide and amine, a resin having the following structural formula can be illustrated: 
wherein R1, R2 and B are as follows, and n is an integer of 3 or 4.
If the amount of the cationic electrodeposition synthetic resin having an amino group used is less than 40 percent by weight, mechanical properties of the dried coating layer are weak, and if the amount of the cationic electrodeposition synthetic resin having a amino group used exceeds 60 percent by weight, the manufacture of a water-dispersed solution is difficult. Therefore, the amount of the cationic electrodeposition resin having an amino group used is preferably about 40-60 percent by weight, and more preferably about 45-55 percent by weight.
In addition, as an other cationic electrodeposition resin, an acrylic cationic electrodeposition resin having an amino group which is copolymerized with butyl acrylate, methyl acrylate, hydroxy ethyl acrylate, styrene, methyl methacrylate, (N,N-dimethyl)aminoethyl methacrylate, etc., can be illustrated. The structural formula of the acrylic cationic electrodeposition resin is as follows: 
If the amount of the acrylic cationic electrodeposition resin used is less than 5 percent by weight, a dried coating film having an epoxy-acrylic double-layered structure cannot be obtained, and if the amount of the acrylic cationic electrodeposition resin used exceeds 10 percent by weight, the epoxy resin and acrylic resin are not compatible, thereby affecting the surface appearance and gloss. Therefore, the amount of the acrylic cationic electrodeposition resin used is preferably about 5-10 percent by weight, and more preferably about 7-9 percent by weight.
Usually these resins having an amino group are used together with a blocked polyisocyanate crosslinking agent. The isocyanate is so completely blocked that it can be mixed with the resin having an amino group. Otherwise, the isocyanate is partially blocked in such a manner that it can react with the backbone of a resin. As for the blocked polyisocyanate crosslinking, agent, a compound having the following structural formula can be illustrated: 
If the amount of the blocked polyisocyanate crosslinking agent used is less than 30 percent by weight, properties such as pencil hardness, corrosion resistance, etc., are diminished. If the amount of the blocked polyisocyanate crosslinking, agent used exceeds 50 percent by weight, the preparation of a water-dispersed solution is difficult and properties such as impact-resistance, flexible-resistance, etc., are diminished. Therefore, the amount of the blocked polyisocyanate crosslinking agent used is preferably about 30-50 percent by weight, and more preferably 35-45 percent by weight.
Particularly, the composition of the present invention comprises a fatty acid ester synthetic resin synthesized by an estrification reaction of styrene-allylalcohol and fatty acid in order to improve a smoothness of the electrodeposition coating film and to inhibit pinhole and cratering. As the fatty acid ester synthetic resin, a resin having the following structural formula can be illustrated: 
The molecular weight (weight-average molecular weight) of these fatty acid ester resins is preferably between about 2000 and about 40,000, and more preferably is between about 3000 and about 30,000. If the amount of these fatty acid ester resins used is less than 1 percent by weight, an improvement of the smoothness of the electrodeposition coating film and the restraint of pinhole and cratering cannot be achieved. But, if the amount of these fatty acid ester resins used exceeds 3 percent by weight, mechanical proper ties of the dried coating film such as pencil hardness is deteriorated. Therefore, the amount of the fatty acid ester resin used is preferably about 1-3 percent by weight, and more preferably about 1.5-2.5 percent by weight.
The aqueous medium may contain a coalescing solvent besides water. As an effective coalescing solvent, ketone, hydrogen carbonate, alcohol, ether, etc. can be illustrated. The cationic electrodeposition composition according to the present invention may contain pigments, solvents and additives such as a surface improving additive, a surfactant, a thickening agent, a catalyst, etc. As the catalyst, a metallic salt of organic acids such as cobalt acetate, chromeoctoate, chromenaphthenate, lead acetate and dibutyltinoxide may be usually used.
In accordance with the present invention, the cationic electrodeposition resin can be manufactured in the presence of a low boiling point organic solvent by the reaction of (a) about 40-60 percent by weight of a cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction, (b) about 5-10 percent by weight of an acrylic cationic electrodeposition resin having an amino group, (c) about 1-3 percent by weight of a fatty acid ester resin synthesized by an estrification reaction of styrene-allylalcohol copolymer and fatty acid, and (d) about 30-50 percent by weight of a blocked polyisocyanate crosslinking agent. The low boiling point organic solvent has a boiling point less than approximately 140xc2x0 C. As the low boiling point organic solvent, methylisobutylketone, xylene, tolune, methylethylketone, etc. can be illustrated.
The cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction may be manufactured by using an epoxy resin, polycaprolactonediol, bisphenol A, and benzyl dimethyl amine. Thereafter, about 35-45 parts by weight of the cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 weight parts of an acid for neutralization and about 0.5-1.5 weight parts of a cationic surfactant are mixed to prepare a water-dispersed solution. Then, the water-dispersed solution is extracted under reduced pressure to remove the low boiling point organic solvent. In the following, the water-dispersed solution is filtered to prepare the cationic electrodeposition coating composition. During the process of using the acid for neutralization, the degree of neutralization of the existing amino group is preferably about 20-100%, and more preferably is about 40-70%. According to a suitable neutralization method, an acid for neutralization such as formic acid, acetic acid, lactic acid and phosphoric acid, a surfactant, deionized water, etc., are added and water-dispersed by a high-speed stirring so that excellent water-dispersed resin solution may be obtained.
Successively, a solvent stripping of the low boiling point organic solvent is performed at about 40-80xc2x0 C. and at about 30-90 mmHg, and more preferably is performed under a reduced pressure of about 50-70 mmHg. Then, a high boiling point organic solvent having a boiling point over 140xc2x0 C., which has been included in the preparation of the cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction, the acrylic cationic electrodeposition resin having an amino group, and the fatty acid ester resin synthesized by an estrification reaction of styrene-allylalcohol copolymer and fatty acid, remains. Meanwhile, low boiling point organic solvents such as methylisobutylketone, xylene, tolune, methylethylketone, etc., are partially removed.
After the solvent stripping process for making the solid content of the cationic electrodeposition water-dispersed resin solution about 20-60 percent by weight, and more preferably about 30-40 percent by weight, the water-dispersed resin is filtered through diatomaceous earth in such a manner that an improved cationic electrodeposition coating composition can be obtained. The average particle size of the improved cationic electrodeposition coating composition is less than 0.3 xcexcm and the organic solvent content is minimized.
Also, the present invention provides a pigment grinding vehicle for the cationic electrodeposition coating composition. The pigment grinding vehicle is prepared by using about 25-35 percent by weight of polyglycidylether of bisphenol A, about 5-12 percent by weight of propyleneglycol monomethyl ether acetate, about 8-15 percent by weight of a partially blocked isocyanate cross-linking agent, about 25-40 percent by weight of ethyleneglycol monobutyl ether, about 10-20 percent by weight of an organic tertiary aminate and about 0-5 percent by weight of deionized water.
If the amount of the polyglycidylether of bisphenol A used is less than about 25 percent by weight, properties such as corrosion resistance, water resistance, etc. are deteriorated, and if the amount of the polyglycidylether of bisphenol A used exceeds 35 percent by weight, mechanical properties of the dried coating layer such as hardness is diminished. Therefore, the amount of the polyglycidylether of bisphenol A used is preferably about 25-35 percent by weight, and more preferably about 27-32 percent by weight.
If the amount of the propyleneglycol monomethyl ether acetate used is less than 5 percent by weight, the process of melting the polyglycidylether resin of bisphenol A is poorly performed. If the amount of the propyleneglycol monomethyl ether acetate used exceeds 12 percent by weight, the stability of the paint manufactured with the pigment grinding vehicle is deteriorated. Therefore, the amount of the propyleneglycol monomethyl ether acetate used is preferably about 5-12 percent by weight, and more preferably about 7-10 percent by weight.
If the amount of the partially blocked isocyanate cross-linking agent used is less than 8 percent by weight, the hardness of the coating layer is lessened. If the amount of the partially blocked isocyanate cross-linking agent used exceeds 15 percent by weight, the mechanical property of the coating layer is lowered due to the increase of the hardness of the coating layer. Therefore, the amount of the partially blocked isocyanate cross-linking agent used is preferably about 8-15 percent by weight, and more preferably about 10-13 percent by weight.
If the amount of the ethyleneglycol monobutyl ether used is less than 25 percent by weight, the stability of the electrodeposition paint is poor, and if the amount of the ethyleneglycol monobutyl ether used exceeds 40 percent by weight, the organic solvent content is increased. So, the amount of the ethyleneglycol monobutyl ether used is preferably about 25-40 percent by weight, and more preferably about 30-35 percent by weight.
If the amount of the organic tertiary aminate used is less than 10 percent by weight, the water-miscibility is insufficient. If the amount of the organic tertiary aminate used exceeds 20 percent by weight, the property of water resistance is poor. So, the amount of the organic tertiary aminate used is preferably about 10-20 percent by weight, and more preferably about 13-16 percent by weight. At this time, if the polyglycidylether of bisphenol A has an epoxy equivalent weight less than about 900, chemical properties of the coating layer such as water resistance, corrosion resistance, etc., are poor. If the polyglycidylether of bisphenol A has an epoxy equivalent weight larger than about 1200, the appearance of the coating layer and mechanical properties are poor. Therefore, the epoxy equivalent weight of the polyglycidylether of bisphenol A used is preferably about 900-1200 (weight-average molecular weight is 1400-2800).
Hereinafter, the present invention will be explained in more detail referring to the preferred embodiments. However, the present invention is not limited to the following embodiments. The parts are by weight and the percents are by weight too.